In recent years, as a wireless communication system for achieving a high-speed transmission in a limited frequency band, there has been actively studied a multi-user multiple-input multiple-output (MU-MIMO) system obtained by extending a MIMO system, in which a plurality of antennas are installed both in a receiver and a transmitter, so that the space division multiple access (SDMA) may be applied to the MIMO system.
The MU-MIMO is a system form in which a plurality of user terminals each including a plurality of antennas exist for a base station including a plurality of antennas, and simultaneous transmission is carried out for the plurality of terminals in the same wireless frequency band.
A subject of the present invention is an MU MIMO downlink for transmitting a signal from the base station to the respective user terminals. FIG. 1 is an explanatory diagram for illustrating the MU-MIMO downlink, which is the subject of the present invention.
In this case, the number of reception user terminals is indicated by Nusr (Nusr≥2), the number of antennas of a user # u (u=1, 2, . . . , Nusr) is indicated by Nr,u (Nr,u≥1), and the number of antennas of the transmission base station is indicated by Nt (Nt≥1). Therefore, the total number of reception antennas of all the users is given as follows.Nr,total=Nr,1+Nr,2+ . . . +Nr,Nusr 
Moreover, an Nr,u×Nt channel matrix from the transmission base station to the user # u is indicated by Hu. In the MU-MIMO downlink, the signals are simultaneously transmitted from the base station to the respective user terminals. Therefore, reception signals received at the user terminal generally include signals to be transmitted to other users as well as a desired signal. In other words, inter-user interference (IUI) occurs.
It is desired that a countermeasure against the IUI be taken as much as possible at a base station, in which restrictions on a processing amount and the number of the antennas are less strict than those on the user terminal. Thus, in the MU-MIMO downlink, transmission precoding carried out at the base station is indispensable as the countermeasure against the IUI.
As representative precoding for achieving the MU-MIMO downlink, block diagonalization (BD) is widely studied (for example, refer to Non Patent Literatures 1 and 2). The BD method is a precoding method of directing null to users other than a desired user to form a beam space so that the signal is transmitted only to the desired user.
An MU-MIMO environment in which the IUI does not occur at all can be achieved by carrying out this operation for all the users, and a receiver configuration in the terminal can consequently be simplified. Meanwhile, the BD method consumes the degree of freedom of the antennas for the IUI removal (nulling), and has thus a problem in that a great reduction in desired power occurs.
To address this problem, there is proposed a nonlinear-BMD (NL-BMD) method, which combines a block multi-diagonalization (BMD) method of permitting IUI of a part of users to secure a diversity gain and a method of suppressing residual IUI through pre-canceling with each other (for example, refer to Non Patent Literatures 3 to 5).